If a complete failure of normal and emergency coolant flows occurs in a light water reactor, fission product decay would eventually cause melting of the reactor fuel, leading to contact with water. An energetic fuel/coolant interaction (steam explosion) may result. Experiments were performed at Sandia National Laboratories in which ∼5 to 20 kg of molten fuel simulant were delivered into water in which the water mass was 1.5 to 50 times greater than the fuel. These experiments in subcooled and saturated water showed that spontaneous explosions occurred over the range of water/fuel mass ratio and that in certain experiments multiple explosions occurred. The kinetic energy conversion ratio was <2%. A model is proposed to describe the fuel/coolant mixing process. The model is compared to these intermediate-scale experiments. Additional data analysis indicates that the steam explosion is affected by the mixing process.